The Changes of Wetland Network Pattern Associated with Water

1064 Clean – Soil, Air, Water 2012, 40 (10), 1064–1075

Xiaoyun Fan Research Article Baoshan Cui Hui Zhao Zhiming Zhang The Changes of Wetland Network Pattern Associated with Water Quality in the Pearl School of Environment, Beijing Normal University, State Key Joint River Delta, China Laboratory of Environmental Simulation and Pollution Control, Beijing, P. R. China In the last 30 years, water environment and wetland patterns have been experiencing dramatic changes resulted from rapid urbanization, industrialization and population growth in the Pearl River Delta. To investigate the changes of wetland network pattern associated with water quality in this region, the structure indices and intercepting amount of pollutant of wetland network were calculated in this paper. The results showed that there were four type corridor wetland networks, ‘‘inverted V’’, ‘‘#’’, ‘‘main river-way’’, ‘‘man-made ditch’’ according to river wetlands distribution characteristics.

During the period of 1979–2009, river channel indices (Dr, a, b, g) of all the four type networks showed a decreased tendency. For water quality parameter CODMn,NO3 —N, þ NH4 —N, TP, the ‘‘inverted V’’ type had the largest intercepted amount, the second is ‘‘#’’ type, then is ‘‘main river-way’’ type. The ‘‘man-made ditch’’ type also had higher intercepted amount. As to ‘‘inverted V’’ wetland network, amount of pollutants intercepted by per unit area wetland (DAW) was positive correlated with river corridor density, complexity, connectivity, linkage, and near index of paddy field and reservoir. There were significant positive correlations between DAW and corridor density, connectivity, linkage in the ‘‘#’’ wetland network; but significant negative correlation existed between DAW and paddy field density. Similar results also appeared in ‘‘main river-way’’ wetland network besides the significant positive correlation between DAW and near index of reservoir.

Keywords: Distribution characteristics; Reservoir; Structure index; Water environment Received: February 5, 2012; revised: April 3, 2012; accepted: April 10, 2012 DOI: 10.1002/clen.201200050

1 Introduction between wetlands in different regions. Moreover, from 1978 up to now, the PRD has been entering a phase of rapid urbanization and In recent years, urbanization has an important impact on environ- industrialization as a foreland of the reformation and opening in ment worldwide with the growth of economic development and China, which also brings excessive release of pollutants into rivers [1, population [1–6], which mainly involves river water quality 16, 17]. In Guangdong Province, it has been investigated that total deterioration [1, 3, 4], habit quality degradation [2, 5], loss of wetland 8 8 waste and sewage discharge were 67.7 10 t in 2008 and 44.7 10 t structure and function [6, 7], and so on. in 2000, increased about 50% over the past 8 years [18, 19]. And about Wetland is a special ecosystem and interaction zone between 64% of the industrial sewage and 74% of the domestic waste of the terrestrial and aquatic systems [8]. It plays an important role in whole Guangdong Province are discharged into the PRD [20, 21]. hydrologic, geochemical, pollution filtration, erosion control, and Thus, water quality deterioration has been an increasingly serous biological conservation [9, 10]. Thus, changes of wetland network problem in the river system in this region [16, 17]. Now most pattern would influence the wetland function, especially water researches of river water quality and water resource management purification, because wetland has been considered as an important have been mainly focused on methods of external improving river part for better improving regional water quality [11, 12]. water or upgrading and optimization of water treatment facility The Pearl River Delta (PRD) is full of various kinds of water chan- system [1, 4, 17, 22–27]. However, fewer consider the self-purification nels, such as main rivers, streams, ditches which form intricate, and capacity and self-regulation capacity of wetland for water quality. varied river networks [13], and is one of the most complex deltaic With the extensive application of network analysis, Cohen and water systems on the earth [14, 15]. There are different topographies Brown [28] developed a hierarchical network of treatment wetlands by considering only site-level effluent criteria; and they found the designed networks could efficiently enhance overall effectiveness Correspondence: Professor B. Cui, School of Environment, Beijing related to an equal area of uniformly sized wetlands (annual reten- Normal University, State Key Joint Laboratory of Environmental tion improvements of 31% for flow, 36% for sediment, and 27% for Simulation and Pollution Control, No. 19 Xinjiekouwai Street, Beijing 100875, P. R. China phosphorus). Due to mature river network theory, some researchers E-mail: [email protected]; [email protected] have provided many classification methods used to divide different

ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com Clean – Soil, Air, Water 2012, 40 (10), 1064–1075 The Changes of Wetland Network Pattern 1065 river form [29–31]. For example, Howard [29] explored a model for the total land area of Guangdong Province [32]. It is one of the most river networks and divided the river networks into parallel pattern, urbanized and developed regions in China, and there are several arborization, radial pattern, rectangle pattern, gridiron, etc. Stark densely populated cities, such as Guangzhou, Foshan, Zhongshan, [30] suggested that a river network develops by capturing the adja- Zhuhai, Dongguan, and Shenzhen. The PRD is situated in a transi- cent point with the lowest substrate by using an invasion percola- tional zone of the subtropical zone with East Asian monsoon system. tion model. And the multi-scale statistical structure of simulated The mean annual temperature approximately ranges from 14 to and real river networks were investigated via state-of-the-art wavelet- 228C and the mean annual precipitation is approximately 1200– based multi-fractal (MF) formalisms [31], and the results showed that 2200 mm. In the PRD, river wetland distributed broadly is the differences among basins may be the result of distinctly different important wetland type; and is the main corridor wetland which branching topologies in the hill slope versus channel drainage paths. is an important part of wetland network. In this paper we chose Most of these researches were related with the influence of top- river wetlands with centralized urbanization and relative com- ography and geological conditions on river wetland network shapes. pletely wetland ecological system structure as the main study area However, little is considered about pollutant purification function of (Fig. 1). river wetland network. Researches on water purification function of wetlands in regional scale are important pathways to study coordinated development of 2.2 Data sources wetland system and urbanization. Som the objectives of this paper In this study, MSS/TM/ETMþ56 scene remote sensing images of 1979, are mainly: (1) to investigate the spatial distribution characteristics 1986, 1990, 1995, 2000, 2005, 2009 were interpreted to wetland of wetland network; (2) to analyze wetland network change pattern vector data of the time scale for 30 years. from 1979 to 2009; (3) and then to further study the changes of wetland network pattern associated with water quality in the PRD. 2.3 Structure index of river wetland network 2 Study area and methods 2.3.1 Index of corridor 2.1 Study area The basic measurement indices of corridor are as follows: 0 0 The PRD (21840 –238N, 1128–113820 E) is located in Southern China Corridor density (Dr): the total length of river per unit regional with rich river watercourses and wetlands, which occupies 26% of area, this index measures how lengths of rivers develop. The corridor

Figure 1. Map showing the location of Pearl River Delta river water system, the boundary of the study area.

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density can be estimated as: ADi is of the i wetland type:

L Pn Dr ¼ (1) aij A0 j¼1 ADi ¼ 100 (6) A0 where A0 is the total area of region, L is the physical length. Topological structure index (b): the degree of each node is a where aij is the area of single patch in i wetland type, j is the number count of the number of lines connected with a given node. In the of patch in i wetland type, and A0 is the total area in this region. topology, the degree of each given node indicates the accessibility Average patch adjacent index (MPI): and characteristic of network connection for this element. It can be Pn computed as: ai hi MPI ¼ i¼1 (7) L n b ¼ (2) P where MPI is the average adjacent index; n is the number of land- 2 where L is number of connected corridors, P is number of nodes. The scape patch; ai is area of some patch in the region (m ); hi is the value of b ranges from 0 to 3, in that range, the larger b value, the nearest distance from one patch to the similar patch. The smaller the better the network is connected. When b ¼ 0, it indicates there is not value of MPI, the higher the degree of dispersion between patches, corridor. and the higher the fragmentation of the regional landscape is. Connectivity index (g): it denotes the ratio of the number of links in a network to the maximum number of links possible [3], and 2.4 Amount of pollutants intercepted by wetland is related to whether the river corridor is split or obstructed. The corridor wetland with high connectivity will be helpful to display 2.4.1 Amount of regional pollutant discharged into its function stably. It can be computed as: wetland L According to research achievement and pollution source survey data g ¼ ðP 2Þ; ðP 3; P 2 NÞ (3) 3 for many years (Investigation Report of sewage draining outlet into river in Guangdong Province (1993, 1999, 2005); Statistical yearbook where L is the number of lines, P is the number of nodes. The value of of Guangdong Province (1980–2008)), based on investigation of pol- g ranges from 0 to 1, if g ¼ 1, it indicates each node is connected with lution point source and estimation method of non-point sources in the other points; and if g ¼ 0, it indicates each node between each complex river network [34–36], the following methods of investi- other is not connected. gation and statistic analysis were used in this paper. Circuitry index (a): the circuitry of network denotes the extent to which circuits appear in the network. It indicates the optional 2.4.1.1 Investigation of industrial point source pollutant degree for the moving routes of material flow or energy flow. And it can be computed as: According to processing rate, exit amount and distribution conditions of industrial point source pollutants, Tab. 1 showed the total L P þ 1 discharge amount of pollutants in every city. a ¼ ; ðP 3; P 2 NÞ (4) 2P 5 2.4.1.2 Estimation of non-point sources pollution where L is the number of lines, P is the number of nodes. When a ¼ 0, Compared with point source pollution, non-point source pollution there is no circuit in the corridor wetlands; when a ¼ 1, the corridor has characteristics of uncertainty of discharge pathway and emis- wetlands have the most possible circuit number. sions, spatial variability of pollution load, randomness of occurrence time, intermittent of occurrence, complexity of mechanism process, 2.3.2 Index of patches etc. Combined with geomorphology and water system feature of PRD The analysis of patches mainly includes the following content: (1) river network, the transform disciplinarians of non-point source size and type, (2) vegetative structure and diversity, and (3) patch pollutant were generalized as follows. context and naturalness [33]. The basic measurement indices of Fertilizer from paddy field: pollutant amount discharged from patch are as follows. regional upland field and paddy field.

Patch number density (NDi): the ratio of patch number to area. j j It computes the ratio of the total number of patch in all the study Wpi ¼ AiRi (8) area to total area, and the larger the ratio, the higher degree the fragmentation. j where Wpi the amount of j pollutant from i pollution source; Ai area j of upland and paddy ﬁeld; Ri the amount of j pollutant load from i ni pollution source. NDi ¼ (5) A0 2.4.1.3 Fertilizer from fish pond where ni is the number of i patch type, A0 is the regional total area. Patch area density (AD ): the proportion of wetland area in the i j j W ¼ WAiR (9) regional unit area. If A0 is the total area of some wetland type, then pi i

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Table 1. Drainage outlet to rivers of administrative region

City Number of Annual waste water Annual pollutant amount of drainage outlet (104 t/a) drainage outlet amount actual 4 þ actual measured measured (10 t/a) CODMn NO3 –N NH4 –N TP

Guangzhou 128 48 229 1.56 0.13 0.05 0.01 Shenzhen 315 94 492 25.96 9.94 2.15 0.31 Zhuhai 117 11 938 1.48 0.35 0.13 0.02 Dongguan 210 10 4328 0.83 0.17 0.10 0.01 Zhongshan 176 54 788 17.80 5.55 0.49 0.05 Jiangmen 36 17 436 8.44 1.11 0.16 0.02 Foshan 39 20 092 0.42 0.08 0.05 0.01 Total 1021 35 1303 56.50 17.33 3.13 0.43

j topography and climate factors, thus the amount of pollutant from where Wpi the amount of j pollutant from i pollution source; WAi i area of fishpond; Ri equivalent weight of j pollutant from i pollutant stock farming was also not considered in this paper. source in unit area fishpond. In the PRD, sewage was discharged into wetland system mainly by some basic treatment units (sewage treatment plant, drains and 2.4.1.4 Domestic sewage soil). The treatment rates of these treatment units are shown in Tab. 3, and in this paper the median of every treatment rate were used. Table 4 showed the path coefficients of non-point pollutant j j Wpi ¼ NiRi (10) into wetland system which were set following some related research about PRD [25, 37]. j where Wpi the amount of j pollutant from i pollution source, Ni j quantity of i pollution source, Ri the equivalent of j pollutant from i 2.4.1.5 Amount of pollutant into wetland system pollution source. For different pollution sources, the meaning of variable in the formula would be different. When the amounts of Xn pollutant from rural inhabitants were calculated, N was the number e p j i Wi ¼ Wi pið1 fkÞ (11) j i¼1 of rural inhabitants, Ri was the equivalent of pollutant from rural inhabitants. In this paper, the equivalent of every pollutant were e mainly from some researches [35, 36], and then were calibrated by where Wi amount of pollutant from i pollution source into wetland p j water quality data. The value range is shown in Tab. 2. system; Wi amount of pollutant from i pollution source; pi the ratio In addition, the contribution of internal pollution was not of pollutant from i pollution source into wetland system by j path- considered because its influence was very small, especially in way; fk the treated rate of k treated type. Guangzhou district where river channels were dredged every 4 years; moreover, stock farming is underdeveloped in the PRD due to the 2.4.1.6 Amount of pollutant into river cross section

Xn Table 2. Scope of various pollutants equivalent j j Wi ¼ CiQi (12) i¼1 Module CODMn NO3 –N TP NH3–N

Urban population 18.7–28.0 7.5–10.0 0.4–0.6 3.1–5.2 j where Wi amount of j pollutant into wetland system; Qi river ﬂow Rural population 17.3–26.2 7.4–9.8 0.4–0.6 3.1–5.2 j Aquaculture 670.5–1012.8 85.6–114.7 7.9–12.1 14.0–23.4 velocity of river cross section; Ci concentration of j pollutant in input cross section; and i number of input cross section.

Table 3. The scope of treated rates to different treated types

Treatment type Treated rate ( fk) (%)

þ CODMn NO3 –N TP NH4 –N

Wastewater treatment plant 80–90 80–90 80–90 80–90 Drain 3–8 3–7 5–9 3–8 Soil 45–50 45–50 40–45 40–45

Table 4. The path coefficients of non-point pollutant into wetland system zones in the PRD Pollution source Urban life Rural life Upland field Paddy field Fishpond

j Pathway coefficient (Pi ) 0.85 0.70 1.0 1.0 1.0

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2.4.1.7 Amount of pollutant out of river cross section ture index of three type wetland networks were calculated, respectively, and the structural evolutionary characteristics were also Xn analyzed. The results were as follows. W j ¼ CjQ (13) o i i From Figs. 3–5, four river channel indices of ‘‘inverted V’’ and ‘‘#’’ i¼1 network showed a decreased tendency with different degrees, which i where Wo amount of j pollutant out of the wetland system from river indicated the length of river corridor, connectivity, complexity, and j output point; Qi river flow velocity of output cross section; Ci con- linkage were all declined; the corridor index (Dr) of ‘‘main river-way’’ centration of j pollutant of output cross section; and i number of network showed declining tendency, which suggested length of output cross section. corridor was destroyed to some extent; however, a, b, g index were not changed all the time, that was because this type river wetland 2.4.1.8 Amount of pollutants intercepted (DW) were located in the Guangzhou District, and situated in the end of declined state. The D , b, g, a of ‘‘inverted V’’ network ranged from Total mass of pollutants intercepted by wetland system in the region r 0.37 to 0.47, 1.60 to 2.05, 0.55 to 0.75, 0.34 to 0.50, respectively; for based on conventional hydrological conditions in a certain area ‘‘#’’ network, the values of D , b, g, a varied from 0.34 to 0.37, 1.52 to range. It can be computed as: r 1.58, 0.53 to 0.56, 0.29 to 0.36, respectively; to ‘‘main-river’’ network, DW ¼ W1 þ W2 W3 (14) the values extent of these indices were 0.33 to 0.34, 1.42 to 1.43, 0.52 to 0.53, 0.28 to 0.29. where W1 is the input of pollutants from runoff, W2 is the input of Figures 6 and 7 show that the patch index of a three type wetland pollutants from the internal of this region; W3 is the output of network dropped regularly in total. Although there were fluctuate in pollutants from the study region. some years, ND, AW, and MPI of paddy field and reservoir generally decreased in different degree among the three type networks. 2.4.1.9 Amount of pollutants intercepted by per unit area wetland (DAW) 3.3 Water purification function of river wetland Ratio of amount of pollutants intercepted by wetland system to network regional area based on conventional hydrological conditions. It þ Figure 8 shows the DAW of COD, NO3 —N, NH4 —N, TP in four type can be computed as: wetland networks, and the four water quality parameters indicated W1 þ W2 W3 similar change trends. Although there were different pollution DAW ¼ (15) A sources and migration transformation way, the DAW of four water quality parameters were highest in ‘‘inverted V’’ type, then was ‘‘#’’ where A is the total regional area. type, the last was ‘‘main river-way’’ type to the whole regional wetland system, which indicated ‘‘main river-way’’ type declined the 3 Results intercepted pollutant amount of the regional wetland system. 3.1 Spatial morphological analysis of river wetland From 1979 to 1995, capacity of intercepting pollutant of regional wetland network showed declined trend, which increased after 1995, network had some fluctuation tendency; but the DAW was still lower than River network types were generalized and classified based on the that of 1979. Spatially, there was higher capability of intercepting actual shape characteristics of different regional river wetlands pollutants in ‘‘inverted V’’ type; and lower in ‘‘main river-way’’ type. (Fig. 1). According to objective distribution of river wetlands in From the time, three type wetland networks were suffered from the PRD, four type river corridor wetland networks were obtained, different degree influences of a variety of factors from urban sprawl, they were ‘‘inverted V’’ (Fig. 2a), ‘‘#’’ (Fig. 2b), ‘‘main river-way’’ and capability of intercepting pollutant were degraded to varying þ (Fig. 2c), and ‘‘man-made ditch’’ (Fig. 2d). degrees. The DAW were COD > NO3 —N > NH4 —N > TP. The above four type river corridor wetland networks were significant different in space forms, and their shape, structure, develop- 3.4 Relationship between structure indices of river ment and function were also different. From Tab. 5, the ‘‘inverted V’’ wetland network and water purification function networks mainly were located in Dongjiang Region, and the connectivity, circuit index and topologic index were the largest among The intercepting pollutant capability of wetland network is closely the four type networks. The connectivity, circuit index, and topo- related with many factors, such as wetland type, quantity, length of logic index of ‘‘#’’ networks lied in between the values of ‘‘inverted river corridor, and patch area. The structure of wetland network also V’’ and ‘‘main river-way’’. The typical characteristics of ‘‘#’’ networks shows different combination relationship and distribution patterns were straighter river channels, with a 908 intersection and distri- in space. In this paper, the correlations between indices of three type bution by parallel shapes, had good similarity between the whole wetland networks and DAW were analyzed, respectively. The area and parts. The ‘‘main river-way’’ networks had higher percent main density of paddy field was not used to analyze correlation that was rivers, and there was lower connectivity and branching ratio. because it had been used as a basic data for estimation of pollution sources. The correlation analysis results of wetland network indices 3.2 The change trends of river wetland network and DAW are shown in Tab. 6. spatial structure There were closely correlation between wetland network structure and intercepting pollutant function in the PRD. Significant corre-

According to wetland vector data interpreted from 1979, 1986, 1990, lation relationship existed between Dr, g, MPI, and DAW, which 1995, 2000, 2005, 2009 seven remote sensing images, wetland struc- indicated the capability of intercepting pollutant were inﬂuenced

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Figure 2. Sketch maps of four type river corridors; (a) inverted V, (b) #, (c) main-river way, (d) man-made ditches.

by corridor wetlands and patch wetlands. Dr, g, a, MPI–AW1 of three field and reservoir. There were significant positive correlations type wetland networks were all positive correlations. In the between DAW and corridor density, connectivity, linkage in the ‘‘inverted V’’ type, DAW was positive correlated with river corridor ‘‘#’’ type; and significant negative correlation existed between density, complexity, connectivity, linkage and near index of paddy DAW and paddy field density. Similar with the ‘‘#’’ type, the signifi-

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Table 5. Structural features of different types of river corridor

Type of river corridor Inverted V # Main river-way Man-made ditch

Typical region Dong guan City Shunde City, The Guangzhou Nansha District Panyu District, municipal districts Xinhui District Subordinate basin Dongjiang Basin Beijiang and Guangzhou City Reclaimed Xijiang Basin intertidal zone Area of typical district (km2) 1198 914 456 239 Main patch type Paddy field/reservoir Paddy field Paddy field Intertidal zone/mangrove/ fishpond/paddy field Nodes of corridor (Ni)36341750 1 Corridor density (Dr) (km ) 0.382 0.35 0.33 0.78 Topological structure index (b) 1.67 1.53 1.41 1.62 Connectivity index (g) 0.59 0.54 0.53 0.56 Circuit index (a) 0.37 0.30 0.28 0.34 Patch area density (AD) (%) 15.26/2.55 45.39 14.72 1.00/0.56/10.85/38.73 Patch number density (ND) (unit/km2) 0.25/0.15 1.01 0.15 0.26/0.05/0.89/0.97 Average patch adjacent index (MPI) 690/870 250 450 785/890/480/360

cant positive correlation also existed between DAW and river corri- 4 Discussion dor density, connectivity and near degree of reservoir in the ‘‘main river-way’’ type. It can be concluded that river wetland and reservoir 4.1 Spatial morphological analysis of the river always played an important role in intercepting pollutant, so there wetland network were positive correlations among various indices. However, paddy field had some limitations in intercepting pollutant function. If the The topology of river channel and wetland networks can be analyzed percent of paddy field was low in some region, for example, ‘‘main by using some indices of graph theory [33, 38–40]. Linkage effective- river-way’’, ‘‘inverted V’’ wetland network, the closer the layout of ness can be used to evaluate the functional effectiveness of a wetland paddy field (the lower the value of MPI–AW3), the higher the amount network [39], and in a network the circuitry and connectivity are the of pollutant intercepted; otherwise, if the percent of paddy field was important indices to assess the linkage of various elements [33]. In higher in some regions, e.g., ‘‘#’’ wetland network, the looser the this paper, some network topological indices (Dr, a, b, g) were used to layout of paddy field patch (the bigger the value of MPI–AW3), the analyze the structure and characteristics of wetland networks in the higher the wetland water purification capacity was. PRD. The a index for circuitry means what extent the circuits appear

Figure 3. The changing trend of structure indices of ‘‘inverted V’’ river corridors in PRD.

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Figure 4. The changing trend of structure indices of ‘‘#’’ river corridors in PRD. in the network, the g index mainly measures the extent to which feature of this type was close to natural river system with more nodes are connected in a network, which all describe the degree of tributaries with many shapes [32], and was the typical wetland network complexity [33]. structure in estuary region [41]. Moreover, there was slight patch Four type wetland networks ‘‘inverted V’’, ‘‘#’’, ‘‘main-river’’ way, fragmentation compared with other regions. The connectivity, cir- and ‘‘man-made ditches’’ were generalized according to their space cuit index, and topologic index of ‘‘#’’ networks were larger than forms, shape, and structure. The ‘‘inverted V’’ type had the largest those of ‘‘main river-way’’ network. ‘‘#’’ networks were mainly value of three indices (a, b, g) of among the four type networks, located in North river and West river district where paddy field which indicated that there was better and relative complicated was the typical wetland type. The characteristics of ‘‘#’’ networks connectivity. This type network mainly comprised of a group of were straighter river channels, with a 908 intersection and distri- river wetland, reservoir wetland and paddy field. The structure bution by parallel shapes combined with a mass of paddy field patch.

Figure 5. The changing trend of structure indices of ‘‘main river-way’’ corridors in PRD.

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Figure 6. The changing trend of structure indices of paddy field patches in PRD (upper: ‘‘#’’, middle: ‘‘inverted V’’, lower: ‘‘main river-way’’).

Figure 7. The changing trend of structure indices of paddy field and reservoir patches in PRD (upper: ‘‘inverted V’’, lower: ‘‘main river-way’’).

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Figure 8. The changing trend of DAW in different wetland network in 30 years.

The ‘‘#’’ networks were typical wetland network structure influ- different characteristics and degrees of complexity have been used enced by rapid urban sprawl and wetland transfer [42]. The ‘‘main to assess the situation and trends of landscape connectivity and the river-way’’ networks located in Guangzhou District had many main ecological processes associated with it [45–47]. rivers, with lower connectivity and branching ratio. That was In this paper, some network element attributes were assessed because rapid urbanization reduced the quantity of non-primary through wetland patch and river corridor indices analysis to river channel and made the wetland network simple. In Guangzhou investigate the change trends of wetland networks. Figures 4–6 District, urban area accounts for more than 70% of the total area, describe the change trends of the four type wetland networks. and the percentage of river corridor was higher; many medium and The assessment of change trend showed an overall declining small wetlands were replaced by urban land [1, 43]. So the corridor tendency in the river corridor network structure indices for the density and patch density were lower than other wetland types. four type wetland networks except ‘‘main river-way’’ type in ‘‘Man-made ditches’’ networks were a specific wetland network in the period 1979–2009, because many natural wetlands had been PRD reclamation regions with man-made ditch, fishpond, paddy destroyed and removed with increased urbanization [33]. And field, and some mangrove wetlands [32, 44]. the patch indices (ND, AW, and MPI) also dropped regularly on the whole during this period. The smaller the MPI, the lower the 4.2 The change trends of river wetland network proximity is, and patches fragment increasingly. Thus the results indicated that patch of paddy field and reservoirs were reduced spatial structure sharply, not only in quantity but also in area, which induced frag- Network variation can be assessed by analyzing the inherent charac- mentation aggravating. That was because the PRD had experienced teristics of each elements [33]. In recent years, a range of indices with rapid economic development the past three decades, which

Table 6. The correlation between structure indices of river wetland network and its DAW

Type DAW Dr bgaND–AW3 MPI–AW3 AD–AW1 ND–AW1 MPI–AW1

Inverted V CODMn 0.922 0.883 0.925 0.936 0.540 0.937 0.915 0.651 0.867 þ NH4 —N 0.943 0.847 0.892 0.828 0.603 0.701 0.914 0.688 0.849 NO3 —N 0.826 0.812 0.865 0.823 0.525 0.643 0.922 0.653 0.878 TP 0.885 0.878 0.789 0.836 0.693 0.528 0.896 0.584 0.964 # CODMn 0.845 0.705 0.893 0.845 0.620 0.802 –– – þ NH4 —N 0.976 0.549 0.850 0.884 0.544 0.811 –– – NO3 —N 0.986 0.626 0.846 0.864 0.476 0.807 –– – TP 0.965 0.610 0.855 0.873 0.579 0.799 –– – Main river-way CODMn 0.890 0.605 0.893 0.745 0.535 0.816 0.686 0.520 0.836 þ NH4 —N 0.834 0.449 0.850 0.584 0.510 0.803 0.566 0.543 0.826 NO3 —N 0.840 0.526 0.846 0.764 0.435 0.710 0.673 0.456 0.839 TP 0.789 0.573 0.855 0.673 0.549 0.843 0.503 0.479 0.770

AW3: paddy ﬁeld, AW1: reservoir wetland. p 0.05. p 0.01.

ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com 1074 X. Fan et al. Clean – Soil, Air, Water 2012, 40 (10), 1064–1075 have induced numerous ecological and environmental questions should have a suitable size to maintain their integration at land- [48, 49]. scape scale [54], different metrics and size should be chosen according to related study purpose [53].

4.3 Water purification function of river wetland network 5 Conclusion Researching ecological function of wetland in regional scale is an In recent years with the rapid economic development, the whole PRD important tool for studying the development of wetland system and has experienced rapid urbanization and modernization of industry, urbanization. In this paper, the wetland network structure in PRD which induced high population density and rapid development of was closely related with water purification capacity. Higher river industry and agriculture [48, 49]. Thus in this region water environ- corridor connectivity and near index of patch played an important mental quality were greatly influenced by plentiful of waste, exces- role in water purification function of wetland network. Change of sive shoal reclamation, frequent oil spills, over-fishing, etc [43, 49]. wetland pattern in the PRD directly influenced the water purifi- The results showed organic pollutants were the main pollutants cation capability of regional wetland system. With the increasing from four type wetland networks. The pollution sources presented of land use in urban, this influence will also be enlarged. Thus, in their own distribution characteristics in different regions. In ‘‘main order to enhance the water purification capability of wetland net- river-way’’ type located in Guangzhou District, the main pollutant work, not only length of river corridor but also complexity and factors were industrial point source and domestic pollution, and connectivity of river should be considered; meanwhile reservoirs pollutants from farmland were reduced correspondingly for high and paddy field should also be regarded as important factors. urbanization and little paddy and aerobic field. All these phenom- enon contributed to its long history and political position as the capital of Guangdong Province which induced high urbanization Acknowledgments level and urban population density [1]. However, in the ‘‘#’’ region, This research was funded by National Natural Science Foundation of the pollution from fertilizer put into the paddy field occupied a large China (U0833002); China National Funds for Distinguished Young percent of pollution, this was because paddy field was the main Scientists (51125035), and Fundamental Research Funds for the economic sources in this region, and paddy field accounted for more Central Universities (2009SD-24). than 40% of the whole area [42]. ‘‘Inverted V’’ type network were mainly located in the Dongjiang Region where industry and agri- The authors have declared no conflict of interest. culture developed relatively balanced [24], so amounts of pollutants from various pollution sources did not varied considerably. ‘‘Man-made ditches’’ were mainly located in Nansha District where References agriculture and fishery were the chief economic sources with [1] T. P. Ouyang, Z. Y. Zhu, Y. Q. 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